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SubjectRe: [PATCHSET] percpu: generalize first chunk allocators and improve lpage NUMA support
On Wed, Jun 24, 2009 at 05:13:37PM -0700, H. Peter Anvin wrote:
> Andi Kleen wrote:
> >
> > Haven't read the new patches, but per cpu data always was sized
> > for all possible CPUs.
> >
> >> and N is large, what did it cost?
> >
> >> And what are reasonable values of N?
> >
> > N should normally not be large anymore, since num_possible_cpus()
> > is sized based on firmware information now.
> >
>
> *Ahem* virtual machines *ahem*...

And? Even there's not that big typically.

The traditional problem was just for 128 NR_CPUs kernel were nothing
was sized based on machine capacity.

Also on large systems the VMs shouldn't be sized for full capacity.

>
> I have discussed this with Tejun, and the plan is to allocate the percpu
> information when a processor is first brought online (but not removed
> when it is offlined again.) It's a real problem for 32-bit VMs, so it's
> more important than you'd think.

You have to rewrite all code that does for_each_possible_cpu (x)
in initialization then to use callbacks. It would be a gigantic change
all over the tree.

-Andi

--
ak@linux.intel.com -- Speaking for myself only.


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